It has long been known that the heart muscle provides its pumping function in response to electrical events which occur in the atrium and ventricle of the heart. The heart is structured such that conductive tissue connects the atrium and the ventricle and provides a path for the conduction of electrical signals between the two areas. In the operation of a normal heart, a natural atrial event spontaneously occurs in the atrium and a corresponding ventricular event occurs later in the ventricle after a time interval that is typically denoted the AV interval. After the natural occurrence of the ventricular event a new atrial event naturally occurs in the atrium to trigger a succeeding ventricular event. The synchronized electrical events occurring in the atrium and ventricle cause the heart muscle to rhythmically expand and contract and thereby pump blood throughout the body.
In a diseased heart atrial and ventricular events do not naturally occur in the required synchronized manner and the pumping action of the heart is therefore irregular and ineffective to provide the required circulation of blood within the body. The required synchronized activity of such diseased hearts can be maintained by implanting a pacemaker device which applies synchronized stimulating voltage signals to either or both of the atrium and ventricle to pace the heart.
In the early stages of pacemaker development pacemakers were employed to asynchronously stimulate the ventricle of the heart without regard to natural electrical activity occurring in either the atrium or the ventricle. Although this approach had the advantage of simplicity, there was considerable risk due to the fact that paced ventricular events could interact with natural ventricular events to cause the heart to go into a dangerous fibrillation.
As the art of pacing advanced, pacemakers were provided with circuitry which sensed the occurrence of natural ventricular and atrial activity and paced the heart in either the atrium or ventricle only when required to maintain proper operation of the heart.
At the present time it is deemed desirable in some cases to operate a dual chamber pacer in what is known as the DDD mode, wherein electrical events are sensed in the atrium and ventricle and the atrium and ventricle are paced accordingly. Pacers may also be operated in the VDD mode to sense electrical events in the atrium and ventricle and to pace the ventricle. Other pacer modes of operation are employed to sense or pace in either the atrium or ventricle, as required for the particular needs of a heart.
It has been found that pacemakers which operate in the DDD or VDD modes can, under certain circumstances, sustain a dangerous tachycardia condition. A pacer sustained tachycardia condition is defined as an operational pacing state wherein the pacer erroneously stimulates the ventricle of a heart at a dangerously high rate for sustained periods of time.
Pacer sustained tachycardia is initiated when a ventricular event occurs at a time during which the connective tissue between the atrium and ventricle can transmit retrograde electrical signals from the ventricle to the atrium. The conduction of the ventricular signal to the atrium provides a spurious electrical signal in the atrium which appears to the pacer to be a natural atrial event. The pacer senses this spurious retrograde atrial signal and then paces the ventricle at a programmed AV time period following the signal. The paced ventricular signal is conducted to the atrium where it is again erroneously detected by the pacer as a natural atrial event. The pacer therefore continues to pace the ventricle at a relatively high rate defined by the sum of the programmed AV interval and the retrograde conduction time between the ventricle and atrium. This high rate is sustained indefinitely by the pacer, because retrograde conduction ensures that the pacer detects what appear to be high rate atrial events and tracks these spurious atrial events by generating corresponding high rate ventricular paces. This pacer sustained tachycardia condition overstimulates the heart, at considerable danger to the patient.
It is therefore an object of the invention to provide a pacemaker which will operate in a manner that avoids pacer sustained tachycardia.
It is a further object of the invention to provide such a pacemaker that will have a means for breaking out of any pacer sustained tachycardia that occurs.
A pacer which paces the ventricle in accordance with sensed atrial events can dangerously over-stimulate the ventricle by maintaining a high ventricular pacing rate in the presence of a corresponding high natural atrial rate. It has been suggested that this problem may be solved by tracking atrial events and stimulating the ventricle only up to an upper ventricular rate limit value which is programmed for the pacer. When this ventricular rate limit value is reached, the pacer could be programmed to decrease the ventricular stimulation rate in programmed steps to a fallback rate which is slower than the triggering upper ventricular rate.
Thus, the pacer can stimulate the ventricle at an increasing rate which tracks a corresponding naturally increasing atrial rate. However, the ventricle will never be paced at a rate which exceeds a programmed upper ventricular rate limit value and, if the upper rate limit value is ever reached, the ventricular pacing will thereafter slowly decrease in rate so that the patient's heart will not be stimulated at the upper rate for a prolonged period of time.
Although the above-described ventricular rate limiting system provides some measure of safety in controlling the rate of stimulation of the ventricle in response to an elevated atrial rate, it is nevertheless desirable to provide additional means for limiting the ventricular pacing rate. For example, if pacer sustained tachycardia is the cause of the relatively high ventricular pacing rate, even if the rate drops to a fallback level, it is still desirable to break out of the tachycardia and thereby further reduce the rate of pacing of the ventricle. Moreover, if the high ventricular pacing rate is due to an elevated natural atrial rate, it is desirable to enhance pacing efficiency by periodically synchronizing ventricular pacing at an average rate that is less than the ventricular rate limit.
It is also necessary to a ventricular rate limiting system to provide some means for leaving the fallback rate mode if an initially high atrial rate drops to a rate which is lower than the corresponding defined ventricular rate limit. Moreover, if a pacer is operating in a ventricular rate limiting mode, it is advantageous to provide some means for ensuring that, if possible, a natural atrial event will be detected following a paced ventricular event. Thus, unnecessary pacing of the atrium can be avoided.
It is therefore a further object of the invention to provide a pacer with a ventricular rate limiting operation which ensures that the ventricle will be paced at a safe rate in response to high rate atrial events.
Another object of the invention is to provide such a pacer with a ventricular rate limiting mode wherein the ventricle will periodically not be paced in response to relatively high rate atrial events.
A further object of the invention is to provide a ventricular rate limiting system wherein fallback rate operation is discontinued in response to defined low atrial rate activity.
Another object of the invention is to provide a ventricular rate limiting system which ensures that a natural atrial event will be detected following the pacing of the ventricle, if the atrial rate remains high.
A further object of the invention is to provide a ventricular rate limiting pacing system with means for breaking out of a pacer sustained tachycardia.
It has been found that, in atrial pacing systems, it is necessary to turn off or blank the ventricular sense amplifier for a period following pacing in the atrium. This blanking interval is necessary in order to ensure that the ventricular amplifier will not detect the atrial pace signal as a spurious ventricular event. If such a blanking interval is employed, it is necessary to limit the length of the blanking interval in order to ensure that legitimate ventricular events are not ignored.
It has been found that, for a period of time after the end of the blanking interval, a sensed ventricular event has an indeterminate probability of being either a natural ventricular event or a spurious signal. This period in time is denoted the nonphysiological AV interval or artifact sensing interval. If a signal is detected during the nonphysiological AV interval, there is some uncertainty regarding what should be the optimum response of the pacer. It has been suggested that, if a signal is detected during the nonphysiological AV interval, the ventricle should be paced at the end of the interval so that spurious signals will not inhibit the pacing of the ventricle. Moreover, if the detected signal is a real ventricular event, the pacer will not later pace the ventricle during a dangerous T-wave portion of the event. The ventricular pace following a real ventricular event will not capture the ventricle because the ventricular tissue will be refractory.
The suggested nonphysiological pacing scheme has been found to be limiting in that it unnecessarily equates a nonphysiological detection interval with the optimum time for pacing in response to a ventricular signal detected within the interval. It is therefore an object of the invention to provide a pacing system wherein an optimum nonphysiological ventricular pacing time may be defined independently of the end of a nonphysiological signal detection interval.